Add a toggleable Windows 7 (7.css) skin behind a new skin setting

Evaluation branch: a full Aero skin, including frameless windows with our own
title bar, switchable against the existing modern look.

The app already had a four-axis runtime skinning mechanism and zero component
stylesheets, so the skin is one new setting plus one stylesheet section rather
than a parallel UI. The key move is cascade order: 7.css is imported into a
`skin` layer ABOVE Tailwind's `utilities`, so its bare element rules outrank
the ~1900 inline utility classes already in the templates. Template changes are
limited to additive, skin-inert hooks (ARIA roles, `has-scrollbar`,
`w7-toolbar`, `w7-btn`).

- styles.css: explicit layer order, `7.css/dist/7.scoped.css` via `layer(skin)`,
  a token bridge re-pointing the existing semantic tokens at Win7 values (no new
  tokens, so `@theme inline` is untouched), a hand-authored dark Win7 palette,
  and neutralizers that `revert-layer` 7.css's dialog-button metrics back to the
  utility layer. Neutralizers must live inside `skin`, not `skinfix` —
  `revert-layer` only reaches `utilities` from one layer up.
- Frameless chrome: `chromeOptions(skin)` is now the single place either skin's
  frame is decided. `titleBarStyle: "hidden"` on every platform, which is also
  the only way to drop the macOS traffic lights. New `title-bar` component
  mounted by all three window roots.
- Window controls over RPC (minimize/toggle-maximize/close/is-maximized), served
  per window kind so the X reuses each one's existing close path — editors still
  get their dirty prompt. electrobun has no maximize event, so the state is
  derived from `resize`; double-click-to-maximize is implemented here too.
- Font: Noto Sans vendored (OFL-1.1) under ui/src/fonts, since Segoe UI is
  absent off Windows and not redistributable.

Verified: backend `tsc --noEmit` clean, `bun run build:ui` clean (486 kB initial,
under the 500 kB budget), app launches with the skin active and no console
errors. The rendered result has not been eyeballed — screen capture and
accessibility are both denied to the shell on this machine.

Co-Authored-By: Claude <noreply@anthropic.com>

src/shared/types.ts

@@ -78,6 +78,13 @@ export type AccentTone =
 export interface AppSettings {
 	/** UI language; "system" resolves to the OS locale (fallback English). */
 	locale: "system" | SupportedLocale;
+	/**
+	 * Visual skin. "modern" is the native shadcn-style look; "win7" swaps in
+	 * 7.css (Windows 7 Aero) and switches the window to custom frameless chrome.
+	 * Changing this needs a window reopen to take effect on the frame itself —
+	 * the CSS half applies live.
+	 */
+	skin: "modern" | "win7";
 	theme: "system" | "light" | "dark";
 	/** Dark appearance: grey ("dark") or near-black AMOLED ("darker"). */
 	darkTone: "dark" | "darker";
@@ -108,6 +115,7 @@ export interface AppSettings {
 
 export const DEFAULT_SETTINGS: AppSettings = {
 	locale: "system",
+	skin: "modern",
 	theme: "system",
 	darkTone: "dark",
 	accent: "blue",